An External Balance Model for Depopulated Rural Communities: Sensitivity Analysis of Self-Sufficiency Conditions Enabled by AI-Augmented Remote-Income Workers

Rural policy in Japan has long presupposed population recovery, a premise that no longer holds as the nation enters a phase of demographic contraction. This study inverts the question: under what conditions can a depopulated mountainous community of approximately 500 residents sustain a positive external balance without subsidies? We construct a monthly external-balance model (120 months) that treats the community as a minimal open economy, calibrate living-cost parameters with official statistics (National Medical Expenditure estimates, the Survey of Household Expenditure on Education, and the Family Income and Expenditure Survey), and conduct a 1,875-case sensitivity analysis across three axes: per-capita monthly external income of “remote-income workers” — a portfolio comprising remotely employed staff, licensed professionals, AI- augmented contractors, and content/e-commerce operators — the share of such workers in the population, and fixed infrastructure maintenance costs. The baseline scenario (¥400,000/month per worker, approximately equal to the national average salary of ¥4.78 million/year; a 12% worker share; ¥12 million/month infrastructure cost) achieves subsidy-free surplus, as do 84% of all cases. Our principal finding, however, concerns robustness rather than the surplus rate itself: recalibrating living costs with official statistics left the surplus rate nearly unchanged (86% to 84%), revealing that the conclusion is governed by a single unmeasured variable — hamlet-level infrastructure cost, which no public statistics capture because hamlets are not accounting units of local government. We further show that under adverse conditions no single policy lever restores surplus, whereas simultaneous deployment of four levers reverses the balance (from −¥6.45 million to +¥5.90 million/month) through a dynamic synergy that halts the depopulation spiral. The full model is released as open-source software (MIT license), providing a verifiable common basis for rural policy design that does not presuppose population growth. Note: The full manuscript is written in Japanese; an English abstract is provided above.